The present invention relates to a method for producing pentafluorophenyl alkali metal salt with good efficiency using pentafluorobenzene as a raw material therefor.
The pentafluorophenyl alkali metal salt obtainable according to the invention is used as an important reacting agent for introducing pentafluorophenyl group into boron in case it is reacted with boron compounds, for example, boron trichloride etc. to produce boron derivatives which are very useful as co-catalyst ingredients for polymerization reaction, for example, tris(pentafluorophenyl)borane (For example, Synthesis of Fluoroorganic Compounds. p. 190, Springer-Verlag (1985)).
Up to now, several synthetic reactions for pentafluorophenyl alkali metal salt are known. For example, a method of producing pentafluorophenyllithium through bromine-metal exchange reaction using relatively expensive bromopentafluorobenzene as a starting raw material for a source of pentafluorophenyl group and butyllithium is already known. For example, in Synthesis of Fluoroorganic Compounds, p. 190, Springer-Verlag (1985), pentafluorophenyllithium is prepared in diethylether-hexane at -70.degree. C. and is reacted with sulfur dioxide to give lithium pentafluorophenylsulfenate with 94% yield. Also, a method of producing pentafluorophenyllithium through hydrogen-metal exchange reaction using pentafluorobenzene as a starting raw material for a source of pentafluorophenyl group and butyllithium is already known. For example, in J. Org. Chem., 29, 2385 (1964), pentafluorophenyllithium prepared from pentafluorobenzene and butyllithium is reacted with carbon dioxide gas to give pentafluorobenzoic acid wherein the reaction yield is unknown, but the purification yield, depending on reaction solvent system, is 68% in diethylether-hexane system, 80.9% in diethylether type and 82% in diethylether-tetrahydrofuran system respectively.
Further, in J. Org. Chem., 31, 4229, (1966), pentafluorophenyllithium prepared from pentafluorobenzene and butyllithium is reacted with hexafluoroacetone to give undecafluoro-2-phenyl-2-propanol wherein the reaction yield is unknown, but the purification yield is 79%.
Bromopentafluorobenzene has higher reactivity with organometallic compounds such as butyllithium over pentafluorobenzene. When producing pentafluorophenyllithium through hydrogen-metal exchange reaction using butyllithium, the reaction completes for 5 minutes or so in either a chain ether type solvents such as diethyl ether or a cyclic ether type solvents such as tetrahydrofuran to give pentafluorophenyllithium nearly quantitatively. However, since bromopentafluorobenzene is obtained by brominating pentafluorobenzene, it becomes expensive. Eventually a use of more inexpensive pentafluorobenzene would be desired industrially.
However, since pentafluorobenzene is less reactive than bromopentafluorobenzene, when producing pentafluorophenyllithium through deprotonation reaction using butyllithium, it is reported in J. Org. Chem., 29, 2385, (1964) that the yield is more increased in a chain ether type solvent to which a cyclic ether type solvent such as tetrahydrofuran is added than in a chain ether type solvent alone.
In general, it is known that organometallic compounds such as butyllithium react with ether type solvent at a temperature higher than 0.degree. C. Hence, the organometallic compounds such as butyllithium are usually dealt commercially as their solutions in saturated hydrocarbon type solvents such as hexane, cyclohexane, pentane and others. Hence, when producing pentafluorophenyllithium from pentafluorobenzene in chain ether type solvent using butyllithium solution in the saturated hydrocarbon type solvent such as hexane, which is dealt commercially, the saturated hydrocarbon type solvent which is a diluting solvent for butyllithium is mixed into the reaction system to afford a mixed solvent of a chain ether type solvent with a hydrocarbon type solvent, making the reaction further difficult to take place (J. Org. Chem., 29, 2385, ( 1964)).
On the other hand, when producing a compound having very strong Lewis acidity, for example tris(pentafluorophenyl)borane etc., cyclic ether type solvent, it exists in the reaction system, leads to form a complex that the cyclic ether is coordinated in the product by its strong coordinating power, which becomes difficult to be removed therefrom in many cases.
In view of said situation, the inventors have studied various production methods wherein, using pentafluorobenzene as a starting raw material for the source of pentafluorophenyl group in a reaction solvent of not using cyclic ether type solvent such as tetrahydrofuran, pentafluorophenyl alkali metal salt can be produced with good reproductivity in high yield, and, as the result, have reached the present invention.